Supplements for metals (magnesium, zinc, selenium, etc) always list a highly misleading dosage size in milligrams. What you must look for is the elemental part—the actual element in question, not whatever molecule/compound it is being delivered in.
Never use magnesium oxide (not absorbed), or magnesium aspartate (excitotoxin).
When supplementing an essential nutrient like magnesium (Mg), the amount to look for is elemental magnesium NOT the milligrams of the compound.
The same elemental Mg principle applies for all forms, eg magnesium citrate, magnesium L-threonate, magnesium malate, magnesium orotate/etc.
Always look for just the “magnesium” part, which if properly labeled is the actual amount of magnesium itself—elemental magnesium.
However, that’s assuming ~100% absorption. Some forms are far less well absorbed, so that 210mg of elemental magnesium as magnesium citrate might net you as little of 60 to 120mg actually absorbed. In my personal experience, you cannot beat MgCl, but that is disputed.
UPDATE: the science is mixed/inconclusive on Mg bioavailability, with claims the Mg glycinate and Mg malate work best, perhaps better than MgCl. This study on absorption of MgCl claims only 30-50%, a premise I reject because of personal experience with 2000 to 3000mg (2 to 4 grams) elemental Mg per day for months when I was deficient—with no diarrhea. Whatever the true value is, reactions could vary and it might be personal and vary quite a lot with/without food, microbiome, etc. I claim only that MgCl works exceptionally well for me on an empty stomach, and things like citrate give the shits by about 600mg. Use what works for you at a cost that is acceptable. And MgCl is far lower cost than other forms.
I use magnesium chloride (MgCl) for my magnesium source because it is 99% bioavailable, whereas compounds like magnesium citrate could vary from 25% to 70% in bioavailability, varying by physiology, diet, etc. MgCl is the same idea as NaCl (table salt). Your body knows how to these chloride salts instantly and needs the chloride too. Ditto for KCl (potassium chloride). Stick with the 3 chloride salts (NaCl, MgCl, KCl) and you cannot go wrong.
Here, there is 1900mg of magnesium chloride hexahydrate*. But only 210mg of that are magnesium; the other 1690mg is chloride and H20 (water).
Since MgCl is nearly 100% absorbed your dosing is highly reliable—invaluable.
For other forms, you will have take far more of another compound eg magnesium citrate, at far greater cost for the same effective amount available to your body. Worse, the amount of citrate needed might give you toilet splatter. Ignore the speculative marketing hype—barring specific credible evidence otherwise (dubious at best as of 2023), using anything but MgCl is a waste of money sucker’s play.
* Magnesium chloride (MgCl) hexahydrate consists of 1 atom of Mg, 1 atom of Cl, and 6 H20=water molecules.
Nutricost Magnesium Chloride Hexahydrate @AMAZON has exceptionally low lead/arsenic/mercury content according to its COA, about 300X lower than the (lead poisoning) product I had been using. I’ve called the company and their service is superb, they manufacture it all in the USA and they test every batch them make. See the COA for Nutricost Magnesium Chloride Hexahydrate.